1. Field of the Invention
The present invention relates to the production and use of siloxane-containing polyimides, useful as conformal coatings for electrical devices. The polyimides of the present invention are synthesized in a novel Diels-Alder reaction by polymerizing bismaleimide and siloxane-containing symmetrical bis-furan derivatives, and aromatizing the resulting Diels-Alder intermolecular addition product.
2. Description of the Prior Art
The prior art illustrates various attempts to synthesize effective and efficient polyimide coatings for electrical devices. KRAIMAN, U.S. Pat. Nos. 2,890,206, 2,890,207, and CHOW, et al., U.S. Pat. No. 2,971,944 disclose the synthesis of polyimides by the Diels-Alder reaction of bis-maleimides with some dienes, but not with bis-furan derivatives. The polymers obtained from these reactions lack the aromatic structure required for high thermal stability, and the molecular features, such as polydimethyl siloxane segments, needed to impart the requisite flexibility to films and coatings. References that do disclose Diels-Alder syntheses of polymides from bis-furans do not disclose the use of siloxane-modified monomers or polymers. Such references include C. Sheppard, et al., Fabrication of PDA/HTS Prototype Components, 8 Nat'l SAMPE Tech. Conf. (Bicent. Mater.) 257-73 (1976); A. Berlin et al., Poly(aroylene-bisbenzimidazoles) and Polyimides Based on Aromatized Adducts of Bisfurans with Maleic Anhydride, All (1) J. Macromol. Sci.-Chem. 1-28 (1977); AS U.S.S.R. Chem Phys., U.S.S.R. Inventor's Certif. No. SU-584014; M. Mikhailov et al., Polyadducts of Certain Difuran Derivatives, 66 Chem. Abst. 11222n at 1113 (1967); and G. Boyd and K. Heatherington, Synthesis and Reactions of 2-(dialkylamino) furans, 80 Chem. Abst. 47738p at 339 (1974). Thus, in the production of so-called "addition polyimides" via polymerization of maleimide-capped oligomers or Diels-Alder reaction of bismaleimides with dienes, the prior art fails to disclose siloxane-containing reactive dienes of suitable structure. Instead, siloxane-modified addition polyimides have been prepared using siloxane-containing bis-maleimide oligomers such as those disclosed in S. Maudgal and T. St. Clair, Preparation and Characterization of a Siloxane Containing Bismaleimide, Proc. 29th Natl SAMPE Symp. 437-45 (Apr. 3-5 1984). No prior art reference discloses the novel addition polyimides of the present invention, which are produced from polydimethylsiloxane-containing bis-furan derivatives polymerized in a Diels-Alder reaction with bis-maleimides that only optionally contain siloxane segments.
A second widely disclosed class of polyimides, designated as "condensation polyimides," are generally prepared from dianhydrides of tetracarboxylic aromatic acids and diamines via a polyamic acid, or polyamic ester, intermediate. Such condensation polyimides initially may incorporate siloxane segments in a dianhydride, a diamine compound that may be used as a monomer or comonomer in the polycondensation reaction, or both. References disclosing siloxane-containing condensation polyimides include BERGER, U.S. Pat. Nos. 4,030,948; 4,395,527; 4,480,009, and 4,499,149; G. Babu, Silicon-Modified Polyimides: Synthesis and Properties, in Polyimides--Synthesis and Characterization 51-66 (K. Mittal ed. 1982); IGARASHI, U.S. Pat. No. 4,520,075, and HOBACK, U.S. Pat. No. 3,740,305. These references, disclosing silicon-modified condensation polyimides, do not disclose the use of siloxane-containing bis-furans to form silicon-modified addition polyimides.
The objective of the present invention is to provide a versatile process for attaining optimal balances of the properties essential for effective and efficient processing and performance of polyimides used as conformal coatings for electrical devices. By synthesizing siloxane-modified polyimides via the Diels-Alder reaction of bis-maleimides and polydimethyl-siloxane-containing bis-furans, the present invention provides an improved technique for controlling the balance of properties essential for polyimide coatings, including solubility, thermal curability without formation of undesirable by-products, thermo-oxidative stability of cured polymers, electrical properties, moisture resistance, flexibility, and adhesion to substrates.
The present invention solves problems that plagued Polyimide conformal coatings made according to prior-art processes. Specifically, condensation polyimides often are applied to substrates as intermediates, and then cyclized or imidized directly on the substrate. Cyclization of films on the substrate and the necessary synchronization with curing cycles increases the formation of pinholes and undesirable by-products in the cured film. In contrast, the polyimides of the present invention are applied to substrates as fully aromatized end-products, and cured solely by thermal means. By obviating the need to polymerize or cyclize compounds on the substrate, the present invention reduces the incidence of structural and chemical defects in the cured polyimide film.
The present invention also provides an exceptional means for tailoring siloxane-modified polyimide macromolecules to meet the exacting requirements of truly conformal coatings capable of covering the complex geometries of electrical devices. By synthesizing polyimides using the siloxane-containing bis-furan monomers of the present invention, it is possible to control the length of the polydimethyl siloxane segments and, thus, attain a desired flexibility and solubility in the resulting polymer films. Significantly, the present invention also provides for the use of bis-furan monomers in which the silicon atom is linked directly to an aromatic group; this configuration improves the thermal stability of polyimide films. Finally, the structure of the bis-maleimide may also be varied to provide additional opportunities for controlling the properties of the resultant polymer.